1. Field of the Invention
The present invention relates to an objective lens, and more particularly to an objective lens for an optical disc.
2. Description of the Prior Art
An objective lens which is used for treating optical signals on an optical disc is used to record on the disc at high recording density and to reproduce the recorded signals. For practical use with a semiconductor laser, high resolving power on the order of a numerical aperture of 0.45 to 0.5 is needed for an objective lens for reproduction, while high resolving power on the order of N.A. of 0.55 to 0.6 is required for an objective lens for recording. As for an objective lens for reproduction, one for a digital audio disc or one for a video disc has been well known. However, the correction of such off-axis aberration as astigmatism is insufficient generally. As for an objective lens for recording, one for an optical disc file system or one for an optical magnetic disc has been well known. However, due to a great numerical aperture (N.A.) working distance is very small and many lenses are required.